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Oganesson

Oganesson is a synthetic chemical element with symbol Og and atomic number 118. It was first synthesized in 2002 at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia by a joint team of Russian and American scientists. In December 2015, it was recognized as one of four new elements by the Joint Working Party of the international scientific bodies IUPAC and IUPAP. It was formally named on 28 November 2016.[15][16] The name is in line with the tradition of honoring a scientist, in this case the nuclear physicist Yuri Oganessian, who has played a leading role in the discovery of the heaviest elements in the periodic table. It is one of only two elements named after a living person at the time of naming, the other being seaborgium.[17]

Oganesson has the highest atomic number and highest atomic mass of all known elements. The radioactive oganesson atom is very unstable, and since 2005, only five (possibly six) atoms of the nuclide 294Og have been detected.[18] Although this allowed very little experimental characterization of its properties and possible compounds, theoretical calculations have resulted in many predictions, including some surprising ones. For example, although oganesson is a member of group 18 – the first synthetic element to be so – it may be significantly reactive, unlike all the other elements of that group (the noble gases).[2] It was formerly thought to be a gas under normal conditions but is now predicted to be a solid due to relativistic effects.[2] On the periodic table of the elements it is a p-block element and the last one of the 7th period.

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The Danish physicist Niels Bohr was the first to seriously consider the possibility of an element with an atomic number as high as 118, noting in 1922 that such an element would take its place in the periodic table below radon as the seventh noble gas.[19] Following this, Aristid von Grosse wrote an article in 1965 predicting the likely properties of element 118. These were remarkably early predictions, given that it was not yet known how to produce elements artificially in 1922, and that the existence of the island of stability had not yet been theorized in 1965. It was 80 years from Bohr's prediction before oganesson was successfully synthesised, although its chemical properties have not been investigated to determine if it behaves as the heavier congener of radon.[9]

In late 1998, Polish physicist Robert Smolańczuk published calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including oganesson.[20] His calculations suggested that it might be possible to make oganesson by fusing lead with krypton under carefully controlled conditions, and that the fusion probability (cross-section) of that reaction would be close to the lead–chromium reaction that had produced element 106, seaborgium. This contradicted predictions that the cross-sections for reactions with lead or bismuth targets would go down exponentially as the atomic number of the resulting elements increased.[20]

The following year, they published a retraction after researchers at other laboratories were unable to duplicate the results and the Berkeley lab could not duplicate them either.[23] In June 2002, the director of the lab announced that the original claim of the discovery of these two elements had been based on data fabricated by principal author Victor Ninov.[24][25] Newer experimental results and theoretical predictions have confirmed the exponential decrease in cross-sections with lead and bismuth targets as the atomic number of the resulting nuclide increases.[26]

The first genuine decay of atoms of oganesson was observed in 2002 at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, by a joint team of Russian and American scientists. Headed by Russian nuclear physicist of Armenian ethnicity Yuri Oganessian, the team included American scientists of the Lawrence Livermore National Laboratory, California.[27] The discovery was not announced immediately, because the decay energy of 294Og matched that of 212mPo, a common impurity produced in fusion reactions aimed at producing superheavy elements, and thus announcement was delayed until after a 2005 confirmatory experiment aimed at producing more oganesson atoms.[28] On 9 October 2006, the researchers announced[13] that they had indirectly detected a total of three (possibly four) nuclei of oganesson-294 (one or two in 2002[29] and two more in 2005) produced via collisions of californium-249 atoms and calcium-48 ions.[30][31][32][33][34]

In 2011, IUPAC evaluated the 2006 results of the Dubna–Livermore collaboration and concluded: "The three events reported for the Z = 118 isotope have very good internal redundancy but with no anchor to known nuclei do not satisfy the criteria for discovery".[35]

Because of the very small fusion reaction probability (the fusion cross section is ~0.3–0.6pb or 6959300000000000000♠(3–6)×10−41 m2) the experiment took four months and involved a beam dose of 7019250000000000000♠2.5×1019calcium ions that had to be shot at the californium target to produce the first recorded event believed to be the synthesis of oganesson.[36] Nevertheless, researchers were highly confident that the results were not a false positive, since the chance that the detections were random events was estimated to be less than one part in 7005100000000000000♠100000.[37]

In the experiments, the alpha-decay of three atoms of oganesson was observed. A fourth decay by direct spontaneous fission was also proposed. A half-life of 0.89 ms was calculated: 294Og decays into 290Lv by alpha decay. Since there were only three nuclei, the half-life derived from observed lifetimes has a large uncertainty: 6996890000000000000♠0.89+1.07
−0.31 ms.[13]

and checking that the 290Lv decay matched the decay chain of the 294Og nuclei.[13] The daughter nucleus 290Lv is very unstable, decaying with a lifetime of 14 milliseconds into 286Fl, which may experience either spontaneous fission or alpha decay into 282Cn, which will undergo spontaneous fission.[13]

In a quantum-tunneling model, the alpha decay half-life of 294Og was predicted to be 6996660000000000000♠0.66+0.23
−0.18 ms[38] with the experimental Q-value published in 2004.[39] Calculation with theoretical Q-values from the macroscopic-microscopic model of Muntian–Hofman–Patyk–Sobiczewski gives somewhat lower but comparable results.[40]

From 1 October 2015 to 6 April 2016, the Dubna team performed a similar experiment with 48Ca projectiles aimed at a mixed-isotope californium target containing 249Cf, 250Cf, and 251Cf, with the aim of producing the heavier oganesson isotopes 295Og and 296Og. Two beam energies at 252 MeV and 258 MeV were used. Only one atom was seen at the lower beam energy, whose decay chain fitted the previously known one of 294Og (terminating with spontaneous fission of 286Fl), and none were seen at the higher beam energy. The experiment was then halted, as the glue from the sector frames covered the target and blocked evaporation residues from escaping to the detectors. The Dubna team planned to repeat this experiment in 2017.[43] One atom of the isotope 295Og may have been seen in a 2011 experiment at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany aimed at the synthesis of element 120 in the reaction 248Cm+54Cr, but uncertainties in the data meant that the observed chain cannot be definitely assigned to 299120 and 295Og: the data indicates a longer half-life of 295Og of 181 milliseconds than that of 294Og, which is 0.7 milliseconds.[12]

Element 118 was named after Yuri Oganessian, a pioneer in the discovery of synthetic elements, with the name oganesson (Og). Oganessian and the decay chain of oganesson-294 were pictured on a stamp of Armenia issued on 28 December 2017.

Using Mendeleev's nomenclature for unnamed and undiscovered elements, oganesson is sometimes known as eka-radon (until the 1960s as eka-emanation, emanation being the old name for radon).[11] In 1979, IUPAC assigned the systematicplaceholder nameununoctium to the undiscovered element, with the corresponding symbol of Uuo,[44] and recommended it to be used until after confirmed discovery of the element.[45] Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations were mostly ignored among scientists in the field, who called it "element 118", with the symbol of E118, (118), or even simply 118.[3]

Before the retraction in 2002, the researchers from Berkeley had intended to name the element ghiorsium (Gh), after Albert Ghiorso (a leading member of the research team).[46]

The Russian discoverers reported their synthesis in 2006. According to IUPAC recommendations, the discoverers of a new element have the right to suggest a name.[47] In 2007, the head of the Russian institute stated the team were considering two names for the new element: flyorium, in honor of Georgy Flyorov, the founder of the research laboratory in Dubna; and moskovium, in recognition of the Moscow Oblast where Dubna is located.[48] He also stated that although the element was discovered as an American collaboration, who provided the californium target, the element should rightly be named in honor of Russia since the Flerov Laboratory of Nuclear Reactions at JINR was the only facility in the world which could achieve this result.[49] These names were later proposed for element 114 (flerovium) and element 116 (moscovium).[50] However, the final name proposed for element 116 was instead livermorium,[51] and the name moscovium was later proposed and accepted for element 115 instead.[17]

Traditionally, the names of all noble gases end in "-on", with the exception of helium, which was not known to be a noble gas when discovered. The IUPAC guidelines valid at the moment of the discovery approval however required all new elements be named with the ending "-ium", even if they turned out to be halogens (traditionally ending in "-ine") or noble gases (traditionally ending in "-on").[52] While the provisional name ununoctium followed this convention, a new IUPAC recommendation published in 2016 recommended using the "-on" ending for new group 18 elements, regardless of whether they turn out to have the chemical properties of a noble gas.[53]

In June 2016 IUPAC announced that the discoverers planned to give the element the name oganesson (symbol: Og), in honour of the Russian nuclear physicist Yuri Oganessian, a pioneer in superheavy element research for sixty years reaching back to the field's foundation: his team and his proposed techniques had led directly to the synthesis of elements 106 to 113 in cold fusion reactions with lead-208 and bismuth-209 targets, as well as elements 112 through 118 through hot fusion reactions with calcium-48 projectiles.[54] The name became official on 28 November 2016.[17] Oganessian later commented on the naming:[55]

For me, it is an honour. The discovery of element 118 was by scientists at the Joint Institute for Nuclear Research in Russia and at the Lawrence Livermore National Laboratory in the US, and it was my colleagues who proposed the name oganesson. My children and grandchildren have been living in the US for decades, but my daughter wrote to me to say that she did not sleep the night she heard because she was crying.[55]

The stability of nuclei quickly decreases with the increase in atomic number after curium, element 96, whose half-life is four orders of magnitude longer than that of any subsequent element. All isotopes with an atomic number above 101 undergo radioactive decay with half-lives of less than 30 hours. No elements with atomic numbers above 82 (after lead) have stable isotopes.[57] This is because of the ever-increasing Coulomb repulsion of protons, so that the strong nuclear force cannot hold the nucleus together against spontaneous fission for long. Calculations suggest that in the absence of other stabilizing factors, elements with more than 103 protons should not exist. However, researchers in the 1960s suggested that the closed nuclear shells around 114 protons and 184 neutrons should counteract this instability, creating an "island of stability" where nuclides could have half-lives reaching thousands or millions of years. While scientists have still not reached the island, the mere existence of the superheavy elements (including oganesson) confirms that this stabilizing effect is real, and in general the known superheavy nuclides become exponentially longer-lived as they approach the predicted location of the island.[58][59] Oganesson is radioactive and has a half-life that appears to be less than a millisecond. Nonetheless, this is still longer than some predicted values,[38][60] thus giving further support to the idea of this "island of stability".[61]

Calculations using a quantum-tunneling model predict the existence of several neutron-rich isotopes of oganesson with alpha-decay half-lives close to 1 ms.[62][63]

Theoretical calculations done on the synthetic pathways for, and the half-life of, other isotopes have shown that some could be slightly more stable than the synthesized isotope 294Og, most likely 293Og, 295Og, 296Og, 297Og, 298Og, 300Og and 302Og.[38][64] Of these, 297Og might provide the best chances for obtaining longer-lived nuclei,[38][64] and thus might become the focus of future work with this element. Some isotopes with many more neutrons, such as some located around 313Og, could also provide longer-lived nuclei.[65] Since these heavier isotopes greatly facilitate future chemical studies of oganesson, due to their expected longer half-lives, the Dubna team plans to conduct an experiment through the second half of 2017 with a heavier target containing a mix of the isotopes 249Cf, 250Cf, and 251Cf with 48Ca projectiles, aimed at the synthesis of the new isotopes 295Og and 296Og; a repeat of this reaction in 2020 at the JINR is planned to produce 297Og. The production of 293Og and its daughter 289Lv in this reaction is also possible. The isotopes 295Og and 296Og may also be produced in the fusion of 248Cm with 50Ti projectiles, a reaction planned at the JINR and at RIKEN in 2017–2018.[43][66][67]

Oganesson is a member of group 18, the zero-valence elements. The members of this group are usually inert to most common chemical reactions (for example, combustion) because the outer valence shell is completely filled with eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.[68] It is thought that similarly, oganesson has a closed outer valence shell in which its valence electrons are arranged in a 7s27p6configuration.[2]

Consequently, some expect oganesson to have similar physical and chemical properties to other members of its group, most closely resembling the noble gas above it in the periodic table, radon.[69] Following the periodic trend, oganesson would be expected to be slightly more reactive than radon. However, theoretical calculations have shown that it could be significantly more reactive.[7] In addition to being far more reactive than radon, oganesson may be even more reactive than the elements flerovium and copernicium, which are heavier homologs of the more chemically active elements lead and mercury respectively.[2] The reason for the possible enhancement of the chemical activity of oganesson relative to radon is an energetic destabilization and a radial expansion of the last occupied 7p-subshell.[2] More precisely, considerable spin–orbit interactions between the 7p electrons and the inert 7s electrons effectively lead to a second valence shell closing at flerovium, and a significant decrease in stabilization of the closed shell of oganesson.[2] It has also been calculated that oganesson, unlike the other noble gases, binds an electron with release of energy, or in other words, it exhibits positive electron affinity,[70][71] due to the relativistically stabilized 8s energy level and the destabilized 7p3/2 level,[72] whereas copernicium and flerovium are predicted to have no electron affinity.[73][74] Nevertheless, quantum electrodynamic corrections have been shown to be quite significant in reducing this affinity by decreasing the binding in the anion Og− by 9%, thus confirming the importance of these corrections in superheavy elements.[70]

Oganesson is expected to have an extremely broad polarizability, almost double that of radon.[2] By extrapolating from the other noble gases, it is expected that oganesson has a boiling point between 320 and 380 K.[2] This is very different from the previously estimated values of 263 K[75] or 247 K.[76] Even given the large uncertainties of the calculations, it seems highly unlikely that oganesson would be a gas under standard conditions,[2] and as the liquid range of the other gases is very narrow, between 2 and 9 kelvins, this element should be solid at standard conditions. If oganesson forms a gas under standard conditions nevertheless, it would be one of the densest gaseous substances at standard conditions, even if it is monatomic like the other noble gases.

Because of its tremendous polarizability, oganesson is expected to have an anomalously low ionization energy (similar to that of lead which is 70% of that of radon[6] and significantly smaller than that of flerovium)[77] and a standard state condensed phase.[2] Even the shell structure in the nucleus and electron cloud of oganesson is strongly impacted by relativistic effects: the valence and core electron subshells in oganesson are expected to be "smeared out" in a homogeneous Fermi gas of electrons, unlike those of the "less relativistic" radon and xenon (although there is some incipient delocalisation in radon), due to the very strong spin-orbit splitting of the 7p orbital in oganesson.[78] A similar effect for nucleons, particularly neutrons, is incipient in the closed-neutron-shell nucleus 302Og and is strongly in force at the hypothetical superheavy closed-shell nucleus 472164, with 164 protons and 308 neutrons.[78]

The only confirmed isotope of oganesson, 294Og, has much too short a half-life to be chemically investigated experimentally. As such, no compounds of oganesson have been synthesized yet.[28] Nevertheless, calculations on theoretical compounds have been performed since 1964.[11] It is expected that if the ionization energy of the element is high enough, it will be difficult to oxidize and therefore, the most common oxidation state would be 0 (as for the noble gases);[79] nevertheless, this appears not to be the case.[9]

Calculations on the diatomic moleculeOg2 showed a bonding interaction roughly equivalent to that calculated for Hg2, and a dissociation energy of 6 kJ/mol, roughly 4 times of that of Rn2.[2] Most strikingly, it was calculated to have a bond length shorter than in Rn2 by 0.16 Å, which would be indicative of a significant bonding interaction.[2] On the other hand, the compound OgH+ exhibits a dissociation energy (in other words proton affinity of oganesson) that is smaller than that of RnH+.[2]

The bonding between oganesson and hydrogen in OgH is predicted to be very weak and can be regarded as a pure van der Waals interaction rather than a true chemical bond.[6] On the other hand, with highly electronegative elements, oganesson seems to form more stable compounds than for example copernicium or flerovium.[6] The stable oxidation states +2 and +4 have been predicted to exist in the fluoridesOgF2 and OgF4.[80] The +6 state would be less stable due to the strong binding of the 7p1/2 subshell.[9] This is a result of the same spin-orbit interactions that make oganesson unusually reactive. For example, it was shown that the reaction of oganesson with F2 to form the compound OgF2 would release an energy of 106 kcal/mol of which about 46 kcal/mol come from these interactions.[6] For comparison, the spin-orbit interaction for the similar molecule RnF2 is about 10 kcal/mol out of a formation energy of 49 kcal/mol.[6] The same interaction stabilizes the tetrahedral Td configuration for OgF4, as distinct from the square planar D4h one of XeF4, which RnF4 is also expected to have.[80] The Og–F bond will most probably be ionic rather than covalent, rendering the oganesson fluorides non-volatile.[7][81] OgF2 is predicted to be partially ionic due to oganesson's high electropositivity.[82] Unlike the other noble gases (except possibly xenon and radon),[83][84] oganesson is predicted to be sufficiently electropositive[82] to form an Og–Cl bond with chlorine.[7]